Alterations in the hypothalamic-pituitary-thyroid axis after prolonged residence in Antarctica.

The human population which lives and works in polar environments has been increasing steadily over the last 15 years. Very little is known about how these residents adjust to their environment. Cold adaptation in man is a poorly understood phenomenon. Euthermic mammals maintain body temperature during cold exposure via non-shivering thermogenesis, a process which is hormonally mediated. We studied prospectively the response of the hypothalamic-pituitary-thyroid axis in 17 euthyroid men before, during and after assignment to duty in the Antarctic. Serum total and free T4 levels fell slightly but not significantly after very prolonged Antarctic residence. Serum total and free T3 decreased significantly from basal levels of 170 +/- 3 ng/dl and 388 +/- 19 pg/dl to 155 +/- 5 ng/dl and 319 +/- 14 pg/dl respectively after Antarctic duty. Serum T3 levels increased after 42 weeks of polar living, the end of the observation period, but the change did not attain statistical significance. The integrated TSH response to TRH administration increased by 50% to 734 +/- 58 microIU.min/ml over warm climate basal response levels of 456 +/- 33 microIU.min/ml by the end of the study. The daily circadian rhythm of serum cortisol was maintained throughout the study period. The alterations in thyroid hormones which we describe, are apparently related to the chronic cold exposure which our subjects experienced in this polar environment.